1. Field of the Invention
The present invention relates in general to the bulk removal of carbon dioxide from non-acidic gases such as nitrogen and methane, and more particularly to the use of particular cation types of zeolites having the faujasite type of crystal structure as selective adsorbents for the carbon dioxide in a cyclic process wherein desorption is accomplished by a reduction in the bed pressure.
2. The Prior Art
A number of processes for the recovery or removal of carbon dioxide from gas mixtures have heretofore been proposed and practiced on a commercial scale. The processes vary widely, but generally involve some form of solvent absorption, adsorption on a porous adsorbent or diffusion through a semipermeable membrane. The membrane technology is used largely in conjunction with enhanced oil recovery wherein on-site separation of carbon dioxide from methane is carried out in order to provide carbon dioxide for reinjection into the oil-producing formation. In such cases the low capital and equipment costs offset the relatively low CO.sub.2 purity and recovery. The solvent absorption systems utilize either chemical or physical solvents and are effective to selectively remove bulk quantities of CO.sub.2, although energy requirements are increasingly a cause for concern. For purification processes, CO.sub.2 can be effectively removed from gas mixtures containing same using the calcium ion-exchanged form of zeolite A, but because of the strong affinity between the sorbent and adsorbate, thermal energy is required for effective desorption of the CO.sub.2. For bulk separations of CO.sub.2 from gas mixtures it has more recently been proposed to utilize the alkali metal and/or the alkaline earth metal cation forms of zeolites of the faujasite type, i.e., zeolite X and zeolite Y. Such a process is disclosed in European Patent Application No. 0173501 published Mar. 5, 1986, and utilizes either a pressure swing desorption or a non-sorbable purge gas type of adsorbent regeneration.